1. Field of the Invention
One embodiment of the present invention relates to display devices, particularly to electroluminescent display devices. Another embodiment of the present invention relates to electronic devices including such display devices.
2. Description of the Related Art
Various electronic devices such as cellular phones and television receivers include display devices. Electroluminescent (EL) display devices using electroluminescent elements are attracting attention as display devices for their small thickness and light weight.
An EL element in an EL display device is driven in accordance with the amount of the current flowing through the element. For this reason, each pixel in a display portion is connected to wiring for supplying current (current-supplying line). A current-supplying line is composed of a wire extended from outside a display region. Each pixel includes a transistor serving as a switching element for controlling current supplied to the EL element.
Transistors containing polysilicon (p-Si), which have higher field-effect mobility and better electrical characteristics than those of transistors containing amorphous silicon (a-Si), are suitable for the use as switching elements. Transistors containing p-Si, however, are prone to variations in electrical characteristics such as threshold voltage due to bonding defects at grain boundaries.
Accordingly, a structure in which pixels including transistors containing p-Si includes a circuit for compensating variations between transistors in threshold voltage has been disclosed, for example, in Patent Document 1. Patent Document 1 discloses the structure of a display device in which each pixel includes a light-emitting element and a drive transistor as well as two switching parts and two capacitors. The two switching parts include the first to fifth transistors. In other words, in the display device disclosed in Patent Document 1, one pixel includes six transistors.